1. Field of the Invention
The present invention relates to a transmission apparatus used for transmitting electrical signals, optical signals, electric power, etc. between a rotatable body and a fixed body.
2. Description of the Related Art
In the prior art, when transmitting signals between, for example, an automobile steering wheel (rotatable body) and the steering column (fixed body), use has been made of the transmission apparatus such as shown in FIG. 1 and FIG. 2 (Japanese Unexamined Published Utility Model Application No. 62-168581).
This transmission apparatus has a tape-liked flexible cable 11 wound in a mainspring manner between an inner casing 13 and an outer casing 15. The inside end of the flexible cable 11 is affixed to a body portion 13a of the inner casing 13, where it is connected to a lead wire 17, while the outside end of the flexible cable 11 is affixed to a tubular portion 15a of the outer casing 15, where it is connected to another lead wire 19. The inner casing 13 and the outer casing 15 are assembled to be relatively rotatable.
Therefore, for example, if the inner casing 13 is attached to the rotatable body and the outer casing 15 is as attached to the fixed body, signals could be transmitted between the rotatable body and the fixed body in the range of winding and unwinding of the mainspring-like wound flexible cable 11. This type of transmission apparatus is limited in application to the case of reciprocal rotation of the rotatable body, but has no sliding contact portions for electrically connecting, so can constitute a highly reliable transmission route.
In this transmission apparatus, the number of possible relative rotations of the inner casing and the outer casing is determined by the number of turns of the flexible cable, so if desired to increase the number of possible rotations, it is necessary to increase the number of turns of the flexible cable (increase the length). If the number of turns of the flexible cable is increased, however, it becomes difficult to transmit the rotational force of the rotatable body side to the flexible cable as a whole, so, for example, trouble such as reversal and bending of the flexible cable would easily occur.
Therefore, to enable a reduction of the number of turns of the flexible cable, the transmission apparatus shown in FIG. 3 (Japanese Unexamined Published patent Application no. 61-115,751) is proposed. This transmission apparatus has an outer casing 15 and an inner casing 13. An inside end of a flexible cable is affixed at a body portion 13a of the inner casing. The outside end of the flexible cable 11 is affixed to the tubular body 15a of the outer casing. The inside end side of the flexible cable 11 is wound in one direction on the body portion 13a, and the outside end side of the flexible cable 11 is wound inside the tubular body 15a with a winding direction opposite to the inside end side, whereby a portion 11a where the winding direction reversed is provided at an intermediate portion of the flexible cable 11.
According to this apparatus, for example, in the case where the outer casing 15 is affixed and the inner casing 13 is turned in the clockwise direction, the reversing portion 11a of the flexible cable 11 also moves clockwise and the relative rotation of the two casings 13 and 15 is half canceled out, so it is possible to reduce the number of turns of the flexible cable 11 (shorten the length).
This apparatus, however, conversely from the above, suffers from the problem that when the inner casing 13 is rotated in the counterclockwise direction, slack would occur at the flexible cable 11 wound on the inner casing 13 and situations would easily occur where the reversing portion 11a would not move in the counterclockwise direction, so as a result a normal rotational operation is not possible. Therefore, this type of transmission apparatus has not in fact been commercialized.
U.S. Pat. Nos. 3,763,455 and 4,540,223 show electrically connecting apparatuses between a rotatable member and a fixed member, which have spacer means between the outer casing and the inner casing. In accordance with the apparatuses the spacer means can guide the movement of the flexible cable.
However, the apparatuses still have the possibility that the flexible cable can easily move in radial direction between the spacer means and the outside surface of the inner casing or the inside surface of the outer casing, so that the slack of the flexible cable would occur. The apparatuses have no means for pressing the flexible cable against the outside surface of the inner casing or the inside surface of the outer casing.
It is important for transmission apparatus used for automobile that the rotatable member can move easily.